In a known apparatus of this kind (FR-PS No. 1 590 405), an operating element consists of a handwheel which projects from the end of a bearing. The securing member comprises two halves of a clamp that are pulled together by means of two screw connections. The securing member is thereby clamped to the housing of the bearing and, in so doing, locks the handwheel. This construction is costly because two clamping parts and corresponding screw connections are required. Also, the clamping parts must be strong enough to provide operating forces of sufficient magnitude to prevent an adjustment. Nevertheless, it is still possible for unauthorized persons to make an adjustment which can't be detected because the adjustment is facilitated in the course of simply loosening and retightening the screw connections.